1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust pipe of automotive engines to measure the concentration of gas such O2, NOx, or CO, and more particularly to an improved structure of such a gas sensor which is designed to minimize thermal damage to a water-repellent filter.
2. Background Art
Gas sensors are known which are installed in an exhaust pipe of automotive vehicles for use in air-fuel ratio control of engines.
FIG. 7 shows one example of such a type of gas sensor which is taught in Japanese Patent First Publication No. 11-72464 (corresponding to U.S. Pat. No. 6,395,159 B2, issued May 28, 2002).
The gas sensor 9 includes a gas sensor element working to measure the concentration of a given component contained in gasses, a housing within which the gas sensor element is disposed through an insulation porcelain, a gas sensor element protective cover joined to a top end of the housing, and an atmosphere side cover 91 joined to a base end of the housing.
The atmosphere side cover 91 is made up of a main cover 911 and a filter cover 912. The main cover 911 includes a large-diameter portion which is exposed to air and welded to a side wall of the base end of the housing and a small-diameter portion surrounded by the filter cover 912. The filter cover 912 is crimped to form necks which works to joint the filter cover 912 to the small-diameter portion of the main cover 911 through a cylindrical water-repellent filter 23.
An elastic seal 17 is installed hermetically within an open end of the small-diameter portion of the main cover 911. The elastic seal 17 is retained firmly, as clearly shown in the drawing, through the necks of the main cover 911 and the filter cover 912. Leads 16 extend hermetically through holes 170 formed in the elastic seal 17 outside the gas sensor 9 for transmitting a sensor output to and receiving electric power from an external sensor controller.
When the gas sensor 19 is installed in the exhaust pipe of the automotive engine, a top end portion (not shown) of the housing is exposed to the exhaust gasses, so that it is exposed to intense heat. The heat is transmitted to the atmosphere side cover 91, thus causing a top end portion of the main cover 911 to be elevated in temperature. The main cover 911 is, as described above, exposed to air, so that the temperature thereof drops gradually from the top end portion to a base end portion thereof (i.e., an upper portion, as viewed in the drawing).
The small-diameter portion of the main cover 911 is surrounded by the water-repellent filter 23 and the filter cover 912, so that the heat stays at the small-diameter portion.
The water-repellent filter 23 is usually made of a porous resin material, so that it may, like the elastic seal 171, undergo a thermal deformation or a change in coefficient of elasticity, thus causing the joint of the water-repellent filter 23 to the main cover 911 to be loosened. The thermal deformation of the water-repellent filter 23 also causes pores thereof to be occluded or closed, thus resulting in disturbance of a flow of the air from outside the filter cover 912 to inside the main cover 911 through air vents 913 and 914. This results in a lack of oxygen within the gas sensor 9, which leads to a failure in operation of the gas sensor 9.